There is a strong movement to increase the energy density in secondary, rechargeable battery systems. The lithium-sulfur (Li—S) battery is one of the most promising approaches to achieve this goal. The abundant element sulfur can reversibly undergo a two-electron reaction per sulfur atom with lithium to afford Li2S, leading to a high theoretical capacity of about 1675 mAh/g, which is an order of magnitude higher than the capacities of currently used transition-metal oxide cathodes. However, through many years of effort, Li—S batteries are still hampered by short cycle life and low electrochemical utilization of the active material. During the discharge reaction with a lithium metal anode, sulfur first forms a series of lithium polysulfides (Li2Sx, x=8, 6, and 4) and then the final discharge products Li2S2 or Li2S. The discharged Li2Sx materials are extremely soluble in the organic electrolytes. Dissolved Li2Sx migrates to the lithium anode and induces shuttling behavior, which consequently results in the loss of active material and capacity degradation. During the past decade, many attempts have been made to encapsulate Li2Sx in the cathode through the development of advanced cathode structures and new electrolytes, which do not dissolve Li2Sx. However, these conventional approaches have not been able to effectively prevent the polysulfide dissolution, migration, shuttling, and loss of active material.
Since the dissolution of Li2Sx and the subsequent consequences could not avoided, the solution to the above problem goes to the other extreme, which is to use a liquid cathode compromising of high concentration Li2Sx dissolved in a mixture solvents such as 1,3-dioxolane a (DOL) and dimethoxyethane (DME). This way, liquid active materials can sufficiently be utilized via the redox reaction on the conductive carbon substrate. In sulfur-carbon composite solid electrodes, without the dissolution of polysulfides, the reduction of nonconductive sulfur can only occur on the sulfur-carbon interface and the bulk sulfur cannot be utilized, resulting in low specific capacity. By taking advantage of soluble polysulfides, recently a particularly interesting approach that uses liquid-phase lithium polysulfide (so-called catholyte) dispersed onto free-standing conductive matrix as the active cathode material has become attractive. This approach shows facile dispersion and homogeneous distribution of the sulfur active material onto the conductive matrix, which is an improvement over solid C—S cathodes. However, migration of lithium polysulfides to the lithium anode can still result in shuttling effects, while Li2S2 and Li2S still can precipitate from the catholyte leading to dramatic capacity loss, as these materials are no longer available for electrochemical cycling. There is an ongoing needs for improved catholyte systems for Li—S batteries. The liquid catholytes and Li—S batteries described herein address this need.